The Biden administration’s push for COVID-19 booster shots has led to questions about why additional doses wouldn’t specifically target the delta strain now ravaging parts of the country.

“Anyone else frustrated that boosters will be the original vaccine?” Dr. Michael Mina, an epidemiologist at Harvard T.H. Chan School of Public Health, wrote on Twitter. “Why push for a vaccine that is already outdated. If we are going to boost — use a delta variant of the vaccine. Delta is our baseline now.”